Tapered Transitional Radial Support for Drilling Tools

ABSTRACT

A bearing assembly for a downhole mud motor has a transitional radial support in the form of a transitionally tapered bearing mandrel. The new radial support includes a generally cylindrical bearing housing having an upper end facing upstream in the direction of the drill string and a lower end, an outer cylindrical surface and an inner cylindrical surface, and wherein a portion of the inner cylindrical surface surrounds and contains a plurality of axial bearings. The bearing mandrel underlies the bearing housing and has an outer surface which defines a lower bearing region. The lower bearing region including a series of stepped outer diameter regions that also define a transitional reduction in cross-section area for the lower bearing region of the mandrel which, in turn, allows the bearing mandrel to have a progressive reduction in bending strength and to become a more active element in bending strength of the mud motor, instead of relying primarily upon the bearing housing.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates generally to earth drilling operationsand, in particular, to an improved radial support which forms a part ofthe bearing section of a mud motor used for such earth drillingoperations.

2. Description of the Prior Art

Mud motors are used for various drilling tasks such straight hole,horizontal and directional oilfield drilling as well as other diverseuses such as river crossings and underground utility drilling. Drillingmotors utilized in drilling oil and gas wells are typically comprised ofthree main components:

-   -   1. A power section that converts the hydraulic power from the        drilling mud being pumped downhole to mechanical power in the        form of rotational torque.    -   2. A transmission that transfers the rotation that is generated        in the power section, usually in the form of eccentric rotation        to the bearing section.    -   3. A bearing section which transfers the rotation generated in        the power section to the drill bit or any downhole equipment        installed bellow the motor. The rotation in the bearing section        is concentric rather than eccentric.

Hunting Energy Services, Houston, Texas, is a leader inprogressive-cavity, positive displacement mud motors. Typical sizesrange from one and eleven sixteenths to nine and five eighths inches,for various applications, be it coil tubing or large hole applications.These include both straight hole and directional applications. Powersections are available in a wide range of speeds.

In a typical oil well drilling application, surface pumps are used tocirculate drilling fluid to flush rock cuttings to the surface fordisposal. The drilling fluid flows down through the bore of the drillstring, exiting into the annulus of the well through the jets in thedrill bit. The cuttings are flushed up the annulus of the well by thereturning drilling fluid. The annulus pressure is substantially lowerthan the drill string bore pressure due to the pressure drop that occursas the drilling fluid passes through the drill bit jets.

The typical mud motor used in these types of drilling operations is aprogressive cavity positive displacement pump (PCPD). As brieflymentioned, the PCPD pump consists of a top sub, which connects the mudmotor to the drill string; the power section, which consists of a rotorand stator; the transmission section, where the eccentric power from therotor is transmitted as concentric power to the bit; the bearingassembly which carries the axial load and radial loads created on thedrilling operations from the bottom sub to the outer housings of themotor; and the bottom sub which connects the mud motor to the bit orother downhole tool. These mud motors use drilling fluid (mud) to createeccentric motion in the power section of the motor which is transferredas concentric power to the drill bit. The mud motor uses different rotorand stator configurations to provide optimum performance for the desireddrilling operation.

A mud motor may also be described in terms of its number of stages, loberatio and external diameter. Stages are the number of full twists thatthe stator makes from one end to the other and the lobe ratio is thenumber of lobes on the stator, to the number of lobes on the rotor (thestator always has one more lobe than the rotor). The operatingparameters include flow rate, bit rpm and torque. The relationshipbetween the rotor and the stator geometry determines the rotationalspeed and torque. The rotational speed is proportional to the flow rateand torque is proportional to the pressure drop in the fluid as it flowsthrough the motor. These principles will be familiar to those skilled inthe relevant mud motor arts.

The particular improvement toward which the present invention isdirected is concerned with the previously mentioned bearing section ofthe assembly. The bearing section is responsible for the axial andlateral control of the assembly. Axial load is usually the weightapplied to the bit (WOB) and sideloads are the result of the welltortuosity and the bend on the motor itself. The bearing section can be“sealed” or “mud lubricated.” The main difference between the two isthat the sealed motors have seals to keep the environment surroundingthe motor from invading the bearing region which is lubricated with oilor grease. A mud lubricated motor uses the drilling mud to lubricate andcool the bearings. Since the lubricating media is not optimal, thebearings themselves may be oversized. The present invention is concernedsolely with the mud lubricated type of bearing assembly.

As will be discussed in more detail in the Detailed Description whichfollows, there are some shortcomings in the present design of the mudlubricated bearing sections of presently available mud motors. Oneaspect, in particular, relates to the shape of the bearing mandrel whichsupports the lower female and male bearings. The shape of the bearingmandrel is determined by the configuration of the radial supportemployed and by the limitations in packaging all of the requiredcomponents inside a housing which has an outside diameter (OD) that isdetermined by the Tool Nominal Size. These constraints have resulted inbearing mandrel designs which have often had a large reduction incross-sectional area in a region of the mandrel between the point ofmain load application and the point of radial support. This reduction incross-sectional area presents a weak point in the prior designs.

Thus, despite the advances that have been made in the art of mud motors,there continues to exist a need for improvements in the component partsof such systems, such as the bearing sections of such drilling motors.

SUMMARY OF THE INVENTION

The improved bearing assembly of the invention features a “taperedtransitional radial support” in the form of a bearing mandrel whichprovides a more transitional reduction in outside diameter (OD) and incross-sectional area than the traditional mandrels employed in the pastin the industry. The result is an improved bearing assembly whichreduces the bending stress on the assembly and spreads the radial loadsin a more effective manner. In addition to improving the transition ofthe bending moment, the improved bearing mandrel of the inventionbecomes a more active element on the bending strength of the assembly,rather than relying solely on the lower bearing housing. Thismodification also allows the use of larger diameter bearings, increasingthe friction surface and reducing the surface pressure on the bearingsthemselves.

In one preferred form, a tapered, transitional radial support for abearing section of a mud motor is shown where the mud motor has a topsub at one end for connection to a drill string and a bottom sub at anopposite end for connection to a drill bit section, a power section, atransmission section and a bearing section which transfers rotationalpower from the power section to the drill bit. The improved radialsupport has a generally cylindrical bearing housing having an upper endfacing upstream in the direction of the drill string and a lower end, anouter cylindrical surface and an inner cylindrical surface, and whereina portion of the inner cylindrical surface surrounds and contains aplurality of axial bearings.

An improved bearing mandrel makes up a portion of the bearing section ofthe assembly. A portion of the bearing mandrel underlies the pluralityof axial bearings and the bearing housing. The bearing mandrel has anupper end for connection with upstream components of the mud motor , alower end for connection to the drill bit section and an open boretherethrough. The bearing mandrel has an outer surface defined betweenthe upper and lower ends thereof which includes a series of steppedouter diameter regions that define a transitional reduction incross-section area and a transitional reduction in outer diameter forthe mandrel. As mentioned, these features allow the bearing mandrel tobecome a more active element on bending strength of the mud motor,instead of relying upon the bearing housing.

A number of other features result from the changes in design of the newbearing section. The transitional reduction in cross-section of thebearing mandrel allows the use of larger diameter bearings, providing anincreased friction surface and reducing surface pressure on thebearings, providing more uniform bearing wear.

In its most preferred form, the new radial support for the bearingsection of a mud motor is provided with a bearing mandrel which has agiven overall length which can be divided into an upper half and a lowerhalf, and wherein there are at least two stepped outer diameter regionson the outer surface of the bearing mandrel, both of which are locatedin the lower half of the outer surface of the bearing mandrel. Mostpreferably, there is a first region of stepped outer diameter which islocated above a second region of stepped outer diameter, the firstregion constituting a greater relative reduction in outer diameter thanthe second region. The second region of stepped outer diameter thencontinues as a constant region of cross-sectional diameter to the upperend of the bearing mandrel.

The improved radial support for the bearing section of a mud motorfeatures a bearing assembly which includes a redesigned lower femalebearing which threadedly engages a lower end of the bearing housing atone extent. The newly designed lower female bearing is intentionallydesigned with decreased wall thickness to accommodate a set of oversizeradial bearings. Preferably, the lower female bearing is a ring-shapedbody with a first cylindrically shaped outer region which steps down toa second outer cylindrical threaded region. The modified shape of thelower female bearing is designed with an internal diameter which takesadvantage of the increase in diameter of the bearing mandrel, whichallows a more uniform spread of load across the length of the entirelower female bearing.

In one preferred arrangement, the first cylindrically shaped outerregion of the lower female bearing overlies a lower male bearing and thesecond outer cylindrical threaded region overlies an intermediate malebearing. The lower male bearing is of greater relative outer diameterthan the outer diameter of the intermediate male bearing. Also, thefirst region of stepped outer diameter of the bearing mandrel forms anexternal shoulder on the outer surface of the bearing mandrel.

Additional objects, features and advantages will be apparent in thewritten description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of the improved bearing section of a mudmotor employing the principles of the invention.

FIG. 2 is a side, cross-sectional view of a prior art bearing section ofa mud motor, showing the principle components thereof and illustratingthe areas which are improved by the design modifications of theinvention.

FIG. 3 is a side, cross-sectional view of the bearing mandrel used inthe prior art bearing assembly of FIG. 2.

FIG. 4 is a side, cross-sectional view of the improved bearing sectionof the invention, showing the relevant components thereof.

FIG. 5 is another side, cross-sectional view, similar to FIG. 3, butillustrating the improved bearing mandrel of the invention.

FIG. 6 is a side cross-sectional view of the female bearing which isused with the improved bearing assembly of the invention.

FIG. 7 is a perspective view, partially broken away, of a prior art mudmotor of the type toward which the improvements of the present inventionare directed, showing the location of the principle operative sectionsthereof.

DETAILED DESCRIPTION OF THE INVENTION

The preferred version of the invention presented in the followingwritten description and the various features and advantageous detailsthereof are explained more fully with reference to the non-limitingexamples included and as detailed in the description which follows.Descriptions of well-known components and processes and manufacturingtechniques are omitted so as to not unnecessarily obscure the principalfeatures of the invention as described herein. The examples used in thedescription which follows are intended merely to facilitate anunderstanding of ways in which the invention may be practiced and tofurther enable those skilled in the art to practice the invention.Accordingly, the examples should not be construed as limiting the scopeof the claimed invention.

Turning first to FIG. 7, there is shown a prior art drilling mud motor,designated generally as 11, of the type toward which the improvements ofthe present invention are directed. There are many varieties of suchtools in use in the drilling industries today, and FIG. 7 is merelyintended to be generally illustrative thereof and to assist in definingthe general environment of the invention. As explained briefly in theBackground discussion, mud motors of this general type are used forvarious drilling tasks such straight hole, horizontal and directionaloilfield drilling, as well as other utility drilling and related tasks.They are connected to the drill string (not shown) to rotate and steerthe drill bit 13. While the description which follows will be centeredaround a “drill bit” such as that used for oil well drilling, it will beunderstood that other types of downhole tools could be attached to themud motor and be powered by the mud motor.

As will be familiar to those skilled in the relevant arts, the typicalmud motor of the type under consideration has a top sub (17 in FIG. 7)which connects the mud motor to the drill string, a power section(generally at 15 which is made up of the rotor 19 and a stator 21), atransmission section (generally at 23) where eccentric power from therotor is transmitted as concentric power to the drill bit, a bearingassembly (generally at 25) carries the axial and radial loads created onthe drilling operations from the bottom sub to the outer housings of themotor, and the bottom sub (generally at 27) which connects the drillingmud motor to the drill bit. Rotation is provided by the power section,which in this case, is a positive displacement motor that is driven bydrilling fluid circulation. Axial and radial drilling loads are directedto the drill string by the bearings within the bearing assembly.

As briefly discussed, in a typical drilling application, surface pumpsare used to circulate the drilling fluid to flush rock cuttings to thesurface for disposal. The drilling fluid flows down through the bore ofthe drill string, exiting into the annulus of the well through the jetsin the drill bit. The cuttings are flushed up the annulus of the well bythe returning drilling fluid. A mud lubricated motor, of the type underconsideration, uses the drilling mud to lubricate and cool the bearings.The present invention is concerned solely with the mud lubricated typeof bearing assembly. Such an improved assembly is shown in perspective,with portions broken away, in FIG. 1 of the drawings.

In order to explain the principles involved in the improved bearingassembly of the invention illustrated in FIG. 1, it may be helpful tofirst review the standard bearing assembly of the type used in the priorart mud motors. Turning first to FIG. 2, a prior art bearing assembly isillustrated. In a typical prior art design, axial bearings 31 arecarried on a bearing mandrel 32. The axial bearings 31are usually heldunder compression inside a main housing 33 which also surrounds thebearing mandrel 32. An internal shoulder 35 provides a stop and asupplementary housing 37 is threaded onto the main housing 33, pressingthe bearing stack and keeping it from spinning. Lower radial support isprovided by a lower female bearing or bushing 39, usually made oftungsten carbide “Tiles” or fused powder.

As has been mentioned, the shape of the bearing mandrel 32 is determinedby the configuration of the radial support and the limitations thatresult from the requirement of packaging all the required componentinside a housing which outside diameter (OD) is determined by the toolnominal size. The main disadvantage of this approach is a significantreduction in cross-sectional area of the prior art bearing mandrel 32between the point of load application (arrow 41 in FIG. 3) i.e., wherethe bit is threaded or the remainder of the bore hole assembly isconnected) and the point of radial support (arrows 43, 45, in FIG.3).

With further reference to FIG. 3, it will be noted that thecross-sectional area of the box end 47 decreases dramatically to asingle, constant area along the remainder of the length of the bearingmandrel 32. A single lower male bearing (sleeve 39 in FIG. 2) wouldtypically surround the bearing mandrel and sit in the region of thearrows 34, 35. In an actual example, the difference in OD between thebox end 47 and the region of the arrows 43, 35, for an 8.00 inch bearingmandrel is 8.00-5.630 inches or 2.370 inches.

The advantages of the invention will now be described, primarily withrespect to FIGS. 4-6. Instead of using a bearing mandrel with a dramaticreduction in cross-sectional area, as has been discussed with respect tothe prior art tool of FIGS. 2-3, the improved bearing mandrel of theinvention (49 in FIG. 4) presents a tapered transition radial supportfor the bearing assembly. In other words, there is a transitionalreduction in cross-sectional area of the mandrel 49 along the length ofthe radial support area.. As a result, in addition to improving thetransition of the bending moment, the new bearing mandrel 49 becomes amore active element on the bending strength of the assembly, instead ofrelying on the lower bearing housing 51. This modification allows theuse of larger diameter bearings (see the lower male bearing 53 and theintermediate bearing 55 in FIG. 4), increasing its friction surface andreducing the surface pressure on the bearings. The new shape of themandrel 49, provides a more transitional reduction in diameter, reducingthe bending stress and spreading the radial loads in a more effectiveway. Note the arrows 75, 77, illustrating the areas where radial loadsare applied in FIG. 5. As seen in empirical results, bearing wear hasbeen more uniform with the new configuration than with the traditionalgeometry.

As illustrated in FIG. 4, the new bearing mandrel 49 has a given overalllength “L1” which can be divided into an upper half and a lower half,and wherein there are at least two stepped outer diameter regions 67,69, on the outer surface of the bearing mandrel. Both of these regionsare located in the lower half of the outer surface of the bearingmandrel below the bisecting line 68. Most preferably, there is a firstregion of stepped outer diameter which is located above a second regionof stepped outer diameter, the first region constituting a greaterrelative reduction in outer diameter than the second region. The secondregion of stepped outer diameter then continues as a constant region ofcross-sectional diameter to the upper end of the bearing mandrel.

The lower male bearing 53 and the intermediate bearing 55 are sleevelike cylinders which surround a portion of the outer surface 57 of thenew bearing mandrel. They are separated by a cross-over piece 59. Theintermediate bearing 55 is situated between the cross-over piece 59 anda catch ring 61. The upper male bearing 53 is situated between thecross-over piece 59 and a thrust ring 63 which abuts an externalshoulder 65 of the bearing mandrel. It will be appreciated that thereare now two lower male bearings, rather than the single male bearing 39of the prior art device. The tapered transitional nature of the newradial support made up by the new bearing mandrel can be appreciatedwith respect to the isolated view of the mandrel 49 shown in FIG. 5.

Note the stepped outer surface regions 67, 69 located below the shoulder71 of the box connection 73, which in this case is a 6.625 APIRegulation Box having an outer diameter of 8.000 inches. In an actualexample, the difference in OD between the box end 73 and the region ofthe first arrows 75 for an 8.50 inch bearing mandrel is 8.000-5.893inches or 2.107 inches, as compared to the difference in OD of the priorart example, which was 2.370 inches. The difference in OD between theregion of the first arrows 75 and the region of the second arrows 77 is5.893-5.003 inches, or 0.890 inches.

The lower male bearing 53 and the intermediate bearing 55 are generallycylindrical sleeve like bodies that are mode of metal carbides,typically tungsten carbide. Since tools used in conjunction with thedrilling of oil and gas wells are subject to considerable abrasion andwear during use, metal carbides are used to form a bearing or wearsurface for downhole tools because of their desirable properties ofhardness, toughness and wear resistance. For example, these radialbearings may have a wear surface which is comprised of a steel supportwhich is inlaid with a layer of solid rectangular tungsten carbide“Tiles”, surrounded by a powered tungsten carbide matrix. This typebearing surface has been found to provide maximum protection againstwear while providing the superior durability necessary for extremeapplications.

As will be appreciated from FIGS. 4, 5 and 6, to accommodate theoversized radial bearings (53 and 55 in FIG. 4) and the new shape of thebearing mandrel 49 itself, a specially designed female bearing 79 wasrequired. The female bearing 79, as shown in FIG. 6 has a generallycylindrical outer surface which steps down to an externally threaded endregion 83. The end region 83 threadedly engages a mating internallythreaded surface of the outer housing 51 (see FIG. 4). The internalprofile of the female bearing 79 also varies in diameter between aregion of greater relative diameter 85 and a tapered region of lesserrelative internal diameter 87. The newly designed female bearing 49overlies the lower male bearing 53 at one end and overlies theintermediate male bearing 55 at the opposite end (see FIG. 4).

The new profile of the female bearing 79 takes advantage of theavailable space to increase the diameter of the lower radial support.The decrease in wall thickness and mechanical strength of this partitself is compensated by the increase in strength of the bearing mandrel49 that allows a more uniform spread of the load across the length ofthe female bearing 79.

The remaining components of the bearing assembly are of conventionaldesign. The bearing housing 51 overlays the axial bearing assembly 89which, in this case, is a series of ball bearings. The assemblyterminates in a flow restrictor/upper female bearing 91 and a flowdiverter/upper radial bearing 93, in the example shown.

An invention has been provided with several advantages. The improvedbearing assembly features oversized radial (male) bearings which can beformed with tungsten carbide Tiles. There are two sets of lower radialbearings instead of one. The maximized diameter of lower radial supportallows for a progressive mandrel cross-sectional diameter transition.The dual radial bearing design improves the mitigation of bending stressmoments through the bearing pack. The new bearing mandrel design allowsthe mandrel to become a more active element on the bending strength ofthe assembly. The modification in design allows the use of largerdiameter bearings, increasing its friction surface and reducing thesurface pressure on the bearings.

While the invention has been shown in only one of its forms, it will beappreciated that it is not thus limited, but is susceptible to variouschanges and modifications without departing from the spirit thereof.

What is claimed is:
 1. A tapered, transitional lower radial support fora bearing section of a mud motor, where the mud motor has a top sub atone end for connection to a drill string and a bottom sub at an oppositeend for connection to a downhole tool such as a drill bit, the mud motoralso having a power section, a transmission section and a bearingsection which transfers rotational power from the power section to thedrill bit, the radial support comprising: a generally cylindricalbearing housing having an upper end facing upstream in the direction ofthe drill string and a lower end, an outer generally cylindrical surfaceand an inner generally cylindrical surface, and wherein a portion of theinner generally cylindrical surface surrounds and contains a pluralityof axial bearings; a bearing mandrel, a portion of which underlies theaxial bearings and the bearing housing, the bearing mandrel having anupper end for connection with upstream components of the mud motor , alower end for connection to the drill bit and an open bore therethrough,the bearing mandrel having an outer surface defined between the upperand lower ends thereof which defines a lower bearing region for thebearing mandrel, the lower bearing region including a series of steppedouter diameter regions that also define a transitional reduction incross-section area for the lower bearing region of the mandrel which, inturn, allows the bearing mandrel to become a more active element inbending strength of the mud motor, instead of relying primarily upon thebearing housing.
 2. The lower radial support for the bearing section ofa mud motor of claim 1, wherein there are multiple regions oftransitional reduction in cross-section area in the lower bearing regionof the bearing mandrel, each of which carries a spaced-apart lower maleradial bearing, the transitional reduction in cross-sectional areaallowing for the use of larger diameter bearings, providing an increasedfriction surface and reducing surface pressure on the bearings,providing more uniform bearing wear.
 3. The radial support for thebearing section of a mud motor of claim 1, wherein the bearing mandrelhas a given overall length which can be divided into an upper half and alower half, and wherein there are multiple regions of transitionalreduction in cross-sectional area in the lower half of the bearingmandrel.
 4. The radial support for the bearing section of a mud motor ofclaim 3, wherein the lower bearing region of the bearing mandrelincludes a lower female bearing with both a lower male bearing and aspaced-apart intermediate male bearing being received within the lowerfemale bearing.
 5. The radial support for the bearing section of a mudmotor of claim 4, wherein there is a first region of stepped outerdiameter on the bearing mandrel which is located above a second regionof stepped outer diameter, the first region allowing a progressivereduction in diameter to the second region.
 6. The radial support forthe bearing section of a mud motor of claim 5, wherein the second regionof stepped outer diameter may or may not continue as a generallyconstant region of cross-sectional diameter to the upper end of thebearing mandrel.
 7. The radial support for the bearing section of a mudmotor of claim 1, wherein the bearing assembly includes a lower femalebearing which threadedly engages a lower end of the bearing housing, thelower female bearing being designed with a relatively constant wallthickness to accommodate oversize radial bearings following an outerprofile dictated by the lower female bearing's outer profile.
 8. Theradial support for the bearing section of a mud motor of claim 7,wherein the lower female bearing is a ring-shaped body with a firstcylindrically shaped outer region which steps down to a second outercylindrical threaded region.
 9. The radial support for the bearingsection of a mud motor of claim 8, wherein the modified shape of thelower female bearing is designed with an internal diameter which takesadvantage of the increase in diameter of the bearing mandrel, whichallows a more uniform spread of load across the length of the entirelower female bearing.
 10. The radial support for the bearing section ofa mud motor of claim 9, wherein the first cylindrically shaped outerregion of the lower female bearing overlies a lower male bearing and thesecond outer cylindrical threaded region overlies an intermediate malebearing.
 11. The radial support for the bearing section of a mud motorof claim 10, wherein the lower male bearing is of greater relative outerdiameter than the outer diameter of the intermediate male bearing.
 12. Aradial support for a bearing section of a mud motor, where the mud motorhas a top sub at one end for connection to a drill string and a bottomsub for connection to a drill bit, a power section including a rotor anda stator, a transmission section where eccentric power from the rotor isconverted and transmitted as concentric power and a bearing sectionwhich transfers rotational power from the transmission to the drill bit,the radial support comprising: a generally cylindrical bearing housinghaving an upper end facing upstream in the direction of the drill stringand a lower end, an outer generally cylindrical surface and an innergenerally cylindrical surface, and wherein a portion of the innergenerally cylindrical surface surrounds and contains a plurality ofaxial bearings; a bearing mandrel, a portion of which underlies theaxial bearings and the bearing housing, the bearing mandrel having anupper end for connection with upstream components of the mud motor, alower end for connection to the drill bit and an open bore therethrough,the bearing mandrel having an outer surface defined between the upperand lower ends thereof which defines a lower bearing region for thebearing mandrel, the lower bearing region including a series of steppedouter diameter regions that also define a transitional reduction incross-section area for the lower bearing region of the mandrel which, inturn, allows the bearing mandrel to become a more active element inbending strength of the mud motor, instead of relying primarily upon thebearing housing; wherein the outer surface of the bearing mandrel has agiven overall length which can be divided into an upper half and a lowerhalf, and wherein there are two stepped outer diameter regions of theouter surface of the bearing mandrel, both of which are located in thelower half of the outer surface of the bearing mandrel, wherein there isa first region of stepped outer diameter which is located above a secondregion of stepped outer diameter, the first region constituting agreater relative reduction in outer diameter than the second region; andwherein a set of lower male bearings are carried within the lowerbearing region on the respective stepped outer diameter regions.
 13. Aradial support for a bearing section of a mud motor, where the mud motorhas a top sub at one end for connection to a drill string and a bottomsub for connection to a drill bit, a power section including a rotor anda stator, a transmission section where eccentric power from the rotor isconverted and transmitted as concentric power and a bearing sectionwhich transfers rotational power from the transmission to the drill bit,the radial support comprising: a generally cylindrical bearing housinghaving an upper end facing upstream in the direction of the drill stringand a lower end, an outer cylindrical surface and an inner cylindricalsurface, and wherein a portion of the inner cylindrical surfacesurrounds and contains a plurality of axial bearings; a bearing mandrel,a portion of which underlies the axial bearings and the bearing housing,the bearing mandrel having an upper end for connection with upstreamcomponents of the mud motor , a lower end for connection to the drillbit and an open bore therethrough, the bearing mandrel having an outersurface defined between the upper and lower ends thereof which defines alower bearing region for the bearing mandrel, the lower bearing regionincluding a series of stepped outer diameter regions that also define atransitional reduction in cross-section area for the lower bearingregion of the mandrel which, in turn, allows the bearing mandrel tobecome a more active element in bending strength of the mud motor,instead of relying primarily upon the bearing housing; wherein the outersurface of the bearing mandrel has a given overall length which can bedivided into an upper half and a lower half, and wherein there arestepped outer diameter regions of the outer surface of the bearingmandrel, all of which are located in the lower half of the outer surfaceof the bearing mandrel; wherein a set of lower male bearings are carriedwithin the lower bearing region on the respective stepped outer diameterregions; and wherein the bearing assembly includes a lower femalebearing which threadedly engages a lower end of the bearing housing, thelower female bearing being designed with a wall thickness designed toaccommodate oversize radial bearings.
 14. The radial support for thebearing section of a mud motor of claim 13, wherein the lower femalebearing is a ring-shaped body with a first cylindrically shaped outerregion which steps down to a second outer cylindrical threaded region.15. The radial support for the bearing section of a mud motor of claim14, wherein the modified shape of the lower female bearing is designedwith an internal diameter which takes advantage of the increase indiameter of the bearing mandrel, which allows a more uniform spread ofload across the length of the entire lower female bearing and a moreuniform bending stress distribution on the mandrel.
 16. The radialsupport for the bearing section of a mud motor of claim 15, wherein thefirst cylindrically shaped outer region of the lower female bearingoverlies a lower male bearing and the second outer cylindrical threadedregion overlies an intermediate male bearing.
 17. The radial support forthe bearing section of a mud motor of claim 16, wherein the lower malebearing is of greater relative outer diameter than the outer diameter ofthe intermediate male bearing.